Summary: We are developing a method for the determination of phosphorus in Haemophilus b polysaccharide conjugate vaccines and pneumococcal polysaccharide vaccines by inductively coupled plasma-atomic emission spectroscopy (ICP-AES). The concentration of polysaccharide is directly related to the concentration of phosphorus measured by ICP-AES. Phosphorus is present in the polyribosylribitol phosphate (PRP) group of the Haemophilus b polysaccharide conjugate vaccine. The Haemophilus b polysaccharide conjugate vaccine is analyzed for phosphorus content within the range of 0.67 to 1.01 ?g phosphorus per 0.5 ml dose. Phosphorus is also present in the polysaccharide component of the pneumococcal polysaccharide vaccines. The vaccine manufacturers have set specifications for phosphorus, in terms of percent composition, for the vaccine. We are developing an improved method and also modifying instrument operating parameters for a newer model ICP-AES to determine the amount of phosphorus present.